“It is the face and a consequence of evil,” he said, adding that the three victims were “all going about their day, all doing what they loved”.

What happened?

The shootings began with a row in the Mercy Hospital car park between the gunman and Dr O’Neal at about 15:00 local time (21:00 GMT) on Monday.

When a friend of the doctor’s tried to intervene “the offender lifted up his shirt and displayed a handgun”. Chicago Police Superintendent Eddie Johnson said.

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Media captionPolice officers line streets in Chicago to honour their colleague

As the friend ran for help, gunfire broke out, according to the Associated Press. One witness, James Gray, told reporters he saw Dr O’Neal fall to the ground and then the gunman “stood over her and shot her three more times”.

When police arrived, the gunman then moved into the hospital, shooting at random and killing pharmacist Dayna Less who was getting off an escalator at the time.

In an exchange of fire with the gunman, police officer Samuel Jimenez was killed. A second police officer avoided injury after a bullet fired in his direction became lodged in his gun in his holster.

What more do we know of the victims?

Dr O’Neal was an emergency physician at Mercy Hospital on Chicago’s South Side. Dr Patrick Connor, director of the hospital’s emergency department, said she had been “dedicated to caring for her community – a wonderful individual”.

Police said the gunman had been in a domestic relationship with Dr O’Neal and media reports say he was her ex-fiance, but this has not been confirmed.

Dr Connor also paid tribute to 25-year-old Dayna Less, saying she had joined the hospital in July as a first-year pharmacy resident and was a recent graduate of Purdue University.

Police officer Samuel Jimenez was a 28-year-old married father. “We cannot thank him enough for his courage and bravery today,” Dr Connor said.

The officer who was shot at but escaped injury during the incident shared images of a bullet embedded in the side of his gun, Chicago news broadcaster ABC7 reported.